It looks to me like it's the front wiring harness and the bulkhead wiring harness that are going to be the focus of the hacking. I'm sure it's not as modular as it appears, but the FSM makes it look like you just need to use the entire WRX engine wiring harness without anything from the older car. I can't find the WRX ECU pinouts right now, but I think most of those were directly engine related.

I think a lot of the 12 hours is not just soldering but achieving the factory look so it doesn't look like a big hack job.